A storage system including a first storage device and a second storage device that have different access speeds has recently been developed. Assume here that the access speed and storage capacity of the first storage device are high and small, respectively, and that the access speed and storage capacity of the second storage device are lower and greater than chose of the first storage device, respectively. The storage system is realized by hierarchically combining the first storage device (hereinafter, referred to as a high-speed storage device) and the second storage device (hereinafter, referred to as a low-speed storage device). Therefore, this storage system is also called a tiered storage system. In the tiered storage system, the high-speed and low-speed storage devices are generally positioned as upper and lower tiers, respectively.
Further, in recent years, in most storage systems, virtualization of a storage area has grown popular. The same can be said of tiered storage systems. In such tiered storage systems, at least parts of the storage areas (hereinafter, referred to as physical volumes; of the high-speed and low-speed storage devices are allocated (i.e., mapped) to a logical volume.
In the tiered storage system, the logical and physical volumes are each divided for management into a plurality of small areas of a certain size. Each of the small areas is called a chunk. If it is necessary to discriminate a chunk in the logical volume from a chunk in the physical volume, the former will be referred to as a logical chunk, and the latter will be referred to as a physical chunk.
A storage controller monitors a status of access to each chunk (for example, each logical chunk). In general, access-frequency statistical values associated with respective chunks and obtained in a recent certain period (monitoring period) are used as an index that indicates the status of access. It is known that as the access-frequency statistical value, a count of accesses (namely, a count of inputs and/or outputs associated with reading and writing; or the total sum of access data amounts is used.
The storage controller relocates, to the high-speed storage device (upper tier), data of a chunk having a great access-frequency statistical value and located on the low-speed storage device (lower tier). Further, the storage controller relocates, to the low-speed storage device, data of a chunk having a low access-frequency statistical value and located on the high-speed storage device. This relocation aims to improve the performance of the entire tiered storage system (namely, the system performance).
However, even when the data of a chunk has been relocated to the high-speed storage device, it is not always expected that the access-frequency statistical value of the chunk is increased in accordance with the difference in access speed between the high-speed and low-speed storage devices. Similarly, even when the data of a chunk has been relocated to the low-speed storage device, it is not always expected that the access-frequency statistical value of the chunk is reduced in accordance with the above-mentioned difference in access speed. That is, when data relocation is performed chunk by chunk, based on the access-frequency statistical value acquired at regular monitoring periods, the system performance may not sufficiently be enhanced.